Electrical controlling apparatus



Aug. 26, 1 w. T. MARCHMENT ET AL 2,603,678

ELECTRICAL CONTROLLING APPARATUS Filed March 14 1949 fatented Aug. 26, 1 952:

VWUNITED STATES PATENT OFFICE-j I 2,608,678 ELECTRICAL CONTROLLING APPARATUS? William Thomas Marchment,

James Reilly Boundy, and Stephen Archbold Bergen, Chis wick, London, England, assignors to Evershedand Vignoles Limited, London, EnglamLa com pany of Great Britain ApplicationMarch 14, 1949, Serial No. 81,352

In Great Britain March 15, 1948 V 14 Claims. (Cl. 318-456) trollers may be used for controlling physical quantities of various kinds as, for example, for maintaining a liquid level, a temperature, a pressure or a degree of humidity in a state of equilibrium and in some kinds of controllers, ac-

tually at or close, to a datum value. The power receiver may control an element such as a valve for controlling the inflow or outflow of a liquid or of;a;moist gas or of a pressure medium, but it may also control the setting of other elements such as a controller of electric current for varia bleheating and so forth. I i

In order to fix ideas, it is convenient to consider the conditions arising in a controller of liquid level. If a controller is designed to effect a setting of the controlled element, for example, a valve controlling 'the outflow of liquid from a tank which is adjusted at all times by an amount proportional to the amount of departure from a datum level and no other factors are utilised, it is clear that. the controller cannot maintainthe datum level because if the inflow to the tank is increased causing the level to rise, the controller is actuated in proportion to that rise in level and has to maintain the outlet valve to -produce equilibrium despite the increased rate of inflow and as the action of the controller is proportional to the increasein level,

it can only establish a new state of equilibrium in ;the system with a new datum at a higher level; similarly, if the inflow is decreased such a: controller can only establish new equilibrium at a lower datum level. Such a form of controller is usually now known as a proportional controller. g

i .If it is required to restore the level of the liquidto the original datum level, it is clear that an additional factor must be impressed upon the controller so that it can maintain a different setting of; the inlet-valve in spite of the fact that; the liquidlevel is to be brought back to the original datum. This additional factor is ofthen inre.otend e M error With spect to time so that controllers depending upon such an additional factor are usually known by the general term of proportional integral controllers. The additional factor may approximate to an integral with respect to time of the actual error or deviation from the datum value at any time,,in which case, the controller is truly a proportional integral controller. However, there are controllers in which the additional factor is of the nature of an integral with respectto time of an empirical error not dependent upon the actual error. tegral controller is sometimes referred to as a proportional resetting controller. I

It is found that proportional integral controllers have a tendency to hunting, that is to say, that the resultant liquid level varies suc-.-

cessively above and below the datumlevel but tending all the time towards this datum value;

In order to overcome this draw-back, it is possible to introduce into the controller a factor ce pending upon the rate of change of the liquidv level with respect to time, and this factor'caus'es any change in liquid level to be opposed. The greater the rate at which the liquid level changes, the greater is this differential or derivative fac tor which therefore assists or opposes the pro portional control according to the sense in which the liquid level is changing. Such a form' of controller, usually called now-a-days a propor tional integral plus derivative controller, is fundamentally a stable form of controller {and the three control factors can be written in'the' standard equation:

Where 0=the deviation or error at any time, and lzoutput from the controller.

The object of the present invention is to pro vide a controller in whichthe first of these factors, that is the proportional factor, and if desired also the integral factor and the derivative factor are all derived from electrical quantities produced by suitable electric and electromagnetic components. I 2

Thus, according to th present invention, the physical quantity to be controlled, such as the level of a liquid in a tank,'is measuredby an electrical transmitter arranged to yield an electric current whose value varies with the magnie tude of the variable physical quantity and the current so obtained is caused to affect directly or indirectly a power receiver connected'to the controlled member, for example, a liquid outlet valve, whose adjustment effects the control of This form of proportional in the variable physical quantity and also the current is employed to establish an additional current component which acts upon the power receiver and thereby causes the latter to operate the controlled member "in the sense, necessary to restore the magnitude of the variable physical quantity towards its datum value. In order to bring the variable quantity to its datum value, the most satisfactory arrangement is to establish an additional current component which represents a true integral with respect to time of the actual deviation of the variable quantity from its datum value. As already mentioned. a controller dependent only upon proportional and integral factors may be unstabl in that excessive correction may cause the state of equilibrium to be overshot and the system to hunt. 'In order to overcome this drawback and cause the system to operate in a stable manner, yet a further component is appliedto the power receiver which component conforms with-the rate of change of the variable quantity and is so applied as to cause the adjustment or the controlled member to be made in the necessary direction to oppose the action of the controller in restoring the variable -quantity to its datum value.

The preferred arrangement for deriving the integ-r'al component includes a pair of photo-electric 'c'ells arranged 'to be difierentially illumihated by a source of light controlled by a shutter actuated by an electrical instrument move"- ment responsive to thecurre'nt derived from the transmitter. If the-photo -electric 'c'ells'are connected as the adjoining arms of a bridge and the output cf the bridge applied to charge the condenserof'an electron discharge tube, the n a -paren s: the tube'canbe made to represent within -practical "limits, the integral with respect to time of the deviation of the variable quantityirom its datum value'and that current is' superimposed 'onthe transmitter cu'rr ish a l ed to .t e'r wer reeeivri'lhen the differential or "derivative "component be obtained by passing-the transmitter curb sal s i m t t al t eya a e p ys uan i y, n by ponn ct ne', a fl e i ii n v rcii acm 's ha i'st m i s l lia ayi 'ai f fierii a aic' r anaem a th r d 'e fgllitfe same lee bn d s u an a; similar cemponerit'appearsin the anode current of the tube.

By "varying the potentialderivedirom the lastmentioned resistance, the constant'of the-differential component maybe varied; and by changing the intensity of the source of illumination iorthe-photo-electric cells, the constant of the integral component may also be varied. This arrangement has the advantage that either may be varied without disturbing or interfering with the adjustment of the. other. h "In order that the invention may, be clearly understood. and-readily carried into effect, three examples "of. controllers in accordance with the invention will now be described with reference to'the accompanying drawing, which is aschematic diagram of "the components and circuit connectionsof 'a preferred form ofth'e invention. "It will be assumed that the-controller is designed to maintain the level or a liquid, which maybe oil, in'a reservoir R. Also in all of "the egamples the 'controlled'member is 'a valve V l i l n'gjt n we mm Wheaten of Ythereservoir s while itis assumed that therate of flower liquidinto thereservoir Rthrough an hrou h ai e stan e s at t eini inlet pipe P is a variable quantity determined by conditions external to the control system so that, in fact, the equilibrium resulting from equal rates of inflow and outflow of liquid is disturbed from time to time so as to cause the level'oi the liquid surface in the reservoir R'to be at least temporarily changed from its datum level which in the drawing is shown at D.

The deviation of the liquid level in the tank R from the datum level is transmitted to establish an electric current which is roportional to the level of the liquid in the tank. The details 'Of'Sll-lOh a transmitter are shown in diagrammatic form.

The transmitter is shown as having parts in two flame-proof casings T, and the circuit arrangementis similar to that disclosed in British Patent, No. 589,423. However, other forms of transmitter may be employed such as that disclosed in British specification No. 265,012. Whateverform the transmitter takes, 'howevenit is operated in accordance 'with the liquid lev'el and provides an output current whose magnitude corresponds to the-liquid level. I

It may be stated briefly that in theform'fllustrated a float F'on the surface-of the'liquid.

in'the tank R, is connected by acord "I tea-pulley 2 which the cord encircles. The cord is weightediat 3 so-thatif the liquid 'lvel'i'ise'sbr falls, 'the pulley 2 is rotated by a corresponding amount. The transmitter also "comprises'the movement of "a moving-coil electrical meter hair ing permanent'magnets '4 and a moving coil'S. The moving coil-5 "and acurreritfindicatin'g m strume'nt 6 are connected in series with the anode circuit of a 'triode electron tube l and its source of high tension supplyas Well efs-l the clistantcr remote apparatus connected by th'e conductors s. 1n the diagram, rue anoee current supply is obtained from the secondary wind-'- mg 9 er transformer in red fro'm l'tefnating current source 1-1, the'eurrentbein'g recitfied by a bridge-connected "rectifier t2. British Patent No. 589,423, another sec'cmdar-y winding l3 of the transformer Hl fed a potential -dividing resistance i l "through a contact rectifier l5. The posit-ive'andngative"terminals of the resistance F; *are connected respectively to two'contacts l6, l-1',-rig-idly supportedrrom-"the pulley zjbut insulated rrcm "one anot er The Iinifl. point Of the resistance |4, bem "the point of zero potential, is can-seeped to the cathodej tt of the 'tri'ode 1. The moving @61115 'beiails a bqii tact arm 1 s which playsbetween' the'contacts-t6, n and is connected tnrdugn'a resistance in to the-cbntrol'grid 2 l of thetriode 1. nger-ail coridenser '22 cemented between 'the 'grid 21 "and cathode l '8"s'erves to "maintain *tl'fe -charge' which the grid-2| receives when the"contact"arm makes contact with lllhr of the contacts 16, H.

f l ne elm-ratmen-sacs ef "the triode 1 and "the values of the resistance 20 aiid condenser 22 are such that the apparatus operates a's follows": 1f the liquid "level in the tank 3 rise the pulley z i'sfrotated in a counterclockwise: 'irection by 'a irdri"espoirdihg"ailiOuiilfiafitithe cofita'ct l 6 litilliies the contact arm l9 and consequently the con-- denser 22 receives "a positive charge 'f-ro-m the resistance 1 4 at a rate nepending "upon the re'sista'n'c 2ft. Thus, the grid 21 is positivelyemerged resulting in the anode current oma e-muesli the c'oil 5 f being increased *"and "the result that the coil 5 is deflected-infer direction to move "the a m 9 i w yi wmit e 1 i*" 5-f f 'asthebontact [6' movesmund followmg the rientact 1'9, but when the contact I6 comes to rest,

the arm I9 floats slightly away from the contact I6 whereupon the charge on the condenser 22 leaks away and contact is again made between I6 and I9 thus causin'gthe arm I9 to float in a position corresponding to that of the contact]? and the corresponding current flowing through the coil can thus be made proportional to the amount that the pulley 2 has been turned'and therei ore tothe liquid level in the tank R. Thus, the operation is very similar to that described 1 British Patent No. 589,423. I In the example shown in the drawing, the anode current of triode I energizes control coil 38 of a known form of electro-pneumaticpower'receiver foroperating the outlet control valve V, although other forms or power device could be used, such as a reversibleelectric motor. The outlet control valve V hasits'rodflfi connected to an air ope'r at-ed diaphragm'illoperated to close the valve-by air pressure above the diaphragmin -a closed chamber 98. This is balanced by a compression spring 89 pressing upwards on the under surface of the diaphragm 81. Coil 38 is carried bypivoted lever 93 and a counter-balancing force is applied to lever 93 by a spring 99 which, upon opening of thevalve V, is stressed by a lever'9I pivoted at 92' and connected to the spring 99 at one end and to the valve rod 86 at the other end, so that as the valve opens, the pull on the pivoted arm 93 increases. I The pressure air is supplied through an inlet pipe 94 to a valve casing 95 and flows to the closed chamber 88 through a pipe 96 controlled by the lower member 91- or a double valve. This valve is actuated by av bellows 98, the upper part of which is carried by a fixed bracket 99, and the lower part of which is attached to thevalve spindle I99. An air nozzle M of fine bore leads from the space within the bellows 98 and air is supplied to the nozzle I9I through a long pipe of fine bore-I 92 leading out of the air supply pipe 94.

The upper valve member I93 on the same spindle es the member 91 controls an outlet from the inside of the valve casing 95 to atmosphere. The end of the pivoted arm 93 is formed as a blade I94 extending over in close proximity to the outlet of the nozzle I9 I. In the balanced condition, air escapes from the nozzle I9 I at a certain speed past the blade I94 but cause the bellows 98 to be partially expanded with the valve members 91, I93, in an intermediate position;-a1- lowing air pressure from the pipe 94 to be partly throttled before reaching the pipe 96 and allowing some air to'escape to atmosphere past.:t'he valve member I93. Thus, the pressure in...the chamber 88 above the diaphragm 81 is balanced by the pressureof the spring 89 and hold thev valve V in the adjusted position.

Assume, however, that the currentin the coil 39 increases,-the lever 93 is rockedcounterclockwise so that the blade I94 moves slightly away from the nozzle IN. This allows the pressure in the bellows 98 to fall somewhat, so that the bellows closes, partially closing the valve member 91 and opening the valve member I93. Therefore, the air supply to the pipe 96 is more fully throttled and there is agreater escape to the atmosphere so that the pressure on the top of the diaphragm 81 decreases and the spring 99 opens the valve V to allow of an increased outflow from the tank R. When the valve v is opened, the lever 9| is also rocked clockwise, the spring 99 stressed to balance the pull of the coil 39.

The anode current from the transmitter flows 6 by way of conductor I and conductor I96 direct.- to the coil 38 and back through the conductor'lIII to the cathode of the triode 1 so that the currentinthe coil 38 is at all times proportional to the liquid level and when it increases, it causes thev valve V to be opened as described above.

In series with the coil 38 is a movement199'oi an electric current measuring instrument consisting of a moving coil with permanent magnets I99. The moving coil carries a light shutter II9. which is controlled in. such a manner that light from a convenient lamp III falls differentially upon a pair of photo-electric cell-s H2, I I 3, in proportion to the output current from the transmitter. The shutter I I9 cooperates with a fixed screen I I4 having apertures I I5, I I6 so that when the liquid level is at the datum'level, the shutter;

M9 is in the central position shown in the draw.- ing and the cells H2, H3 are equally illuminatedthrough the apertures H5, H6 by the lamp I II. If, however, the'liqui-d level falls, the light falling upon the call I I2 is increased by the shutter 'I'I9 moving downwards more fully to open the aperture H5 and the light upon the cell H3 is deicreased in proportion by the shutter II9 partly obscuring the aperture IIS. Conversely, if the liquid level rises, the light falling on the cell M3 is increased relatively to that falling on the cell II2. v

The two photo-electric cells H2, H3 are con.- nected in series to form two arms of a four-arm bridge, the remaining arms of which consist of. resistances H1, H8 of equal value. The centre point of the two cells H2, H3 is connected through a fixed resistance I I9 to one terminal of a grid con-denser I29 and also to the control grid I2 I of a tridoe I22 While the centre point between the resistances H1, H8, is connected to the opposite terminal of the grid condenser I29 and to the cathode I 23 of the triode I22 through a further resistance I24 and a cathode biassing resistance I25. A source of direct potential is con nected across opposite junctions of the bridge and is derived from an alternating current source I26 through a transformer I21 and a rectifier I29.

When the bridge becomes unbalanced, the un-" balance current charges grid condenser I29 to a voltage the value of which is dependent upon the value of the unbalance current and the time during which it flows. The polarity of the charge on condenser I29 will depend upon the direction of.

unbalance of the bridge.

The triode I22 receives its anode potential from an alternating current source I29 through .a. transformer I39 and bridge-connected rectifier I 3I, and its anode current flows by way of conductors I32, I33 through the coi138 in the same direction as the output from the transmitter.

The condenser I29 acts as a storage condenser for integrating the unbalance current. ofv the bridge which is proportional, at any instant, to the deviation of the liquid level in the tank R from the datum level so that the anode current of the triode I22 applied to the coil 38'introduces into the control of the valve V a true integral factor with respect to time. The arrangement of the shutter H9 is such that a fall in the liquid level in the tank R changes the anode current; of the triode I22 so as to close the valve V and conversely the valve tends to be opened whenthe liquid level rises above the datum level. This ad.

ditional integral factor there-fore, tends to restore the liquid level to the datum. When the, datum level is reached, the current from the transmitter .corresponds to :its nominal values causihgthe shutter 1 into: take u its-eentrar sitionrass-howndn the'drawing with the cells :H2, lift-equallyilluminated. However, the igrid;condenser l2ll'lras'acquired a charge-during the peration as zalready'explainedand the an'odefc'ur-' rent of the itriodef H2 ':is; maintained at "a :new

Value which'holds the Valve 'vsalt a setting -:corrcspending to: the new rate of liquid flow into ;-the.

tank :Rrwhich is maintained until the rate of innow througnthe pipe "P 'is again changed.

factor dependingiupon the rate: of change :ofrthe liquid-lev'elwith respeotto time, a differentiatin electrical network is includedin thepath between the rtransmittersand the coil 38 iwhichf'r'esults Finproportional to the liquid leveLflows in this ree.

sistan'ce'. The resistance 1-24 and a condenser I 35 :in-seriesare connec'tedin. parallel with a variable portionof the resistance 134 so that one end ofitheresistance l 354 is connected to the low'er end of the biassing resistance I 25 and the condenser.

I35 is connected to the cathode condenser 1-2 0. i

Clearly, atany instant, the potential across end of the grid the resistance 1-34 is proportional to the liquid. level and the effect of thevdifferentiating network just :described is that a' potential:proportional'to the'first. differential of the current fI'OlIL the,

transmitter is developed across theres-istancei-lM which is in the grid -.ci'rcuit of the "mode" 122.: This results'invarying the anode current of the tube 122 in addition, according. to the rate of change or. the liquid level and the sense of this effect fiStlhIall'ge'd to be such as to reset the outlet valve :V so as to oppose-the change in liquid :Ievel. .A valuable feature circuit constants in, 702;;763, in the equation alreadyv givenmay be varied independently withouttheme affecting the "other :so'thatan adjustment offline constant does not necessita-te any compensation of another.

brightness of the lamp to bevaried. This variation provides. an adjustment of the integral constant 702. The differential. constantks. may be varied'by adjustment'ofthe tapon resistance l 34. A variable resistance '1 31 :isalsoiprovided in shunt tos-coil 3-8 to control the throttling range of the crime system is that the- The lamp H is shown as an-electric lamp with 'a variable resistance 1-36 inqcircuit with its source" of supply enabling the 10, :In order to introduce into the controller. a

controller and this enables thepproportional cone stant' ki to be adjusted.

For convenience, all" the apparatus associatedwith .the movement l H18, the photo-electric icells" lfl '2,"|.='!3', the triodes '1 'an-dil22i and their-confine;- nents. may be enclosed in a flame-proof c'asing similar to the casing T,

We claim:

n-a regulator system in which therha'gnitune-of a physical condition is subject to yarn; tion from a datum 'value 'and in which the magm tune of the-same physical condition may b'e' varied by me-t1 means having a movablemeniber, the

combination {of a transmitter responsive to. said physical condition and pro'ducing an eleetricicurrent whose value varies with them-agnitude of" said condition, a current. operated control device connected to oper'ate saidimovable imenib'er zand;

8'; being :ehergizedhy cmzrent fromsaid transmitter; electric'ineans controlled by {said transmitter for establishing an additional currentcomponent when :thesmagnitude of said1=physica1 condition depairtssfrom fend-datum -value,; said,- additional component conforming with thegint-egral, with ;re spect to time soijthe actual deviationoi the magnitude of :saidphysical condition from its datuin value, said electric meanscornprising abalanced bridge circuit having; two iphoto -cells connected in: adjacent arms thereof, J a condenser connected to be energized through =a -"high resistance iron said bridge,.-a-vacuum tube-relay controlled-by'th voltageacross-said condenser,,and-mcanserespon siveit-o said condition, upon departure thereof fromisaid datum value, to energize said cells ditierenti-ally and thereby unbalance 'said' bride'e andmeans for -supplying said; additional current component energize-said currenteopera'ted control devicein the same direction as" the. current from-said transmitter. electrically-"actuated controller accord ing toclaim -l and including means responsivefio the -magnitude of said condition for applying to the energizing circuitof-said control device a 'fur'-. .thercomponent oi current whi'chlconforms'wf h the rate of changeoi the magnitude of the Va -'.'I able physical condition and is applied iniaflire ti-on pposite -to thecurrcnt ,ironi said trahsr' mitter, I 3. A system according to clainrland including a differentiating circuit. for deriving a voltage conforming witlrthe first differential of the deviation of said transmitter current. ironi as da I tum value, and means' .for superimposing sa'id voltage :upon thecontrol -circuit oi said yacuum" tube relay:..; .;:-Q' s .4; :lna regulator --system in which the mag i tude, ;of- ;a @physical condition issubject; to vane; tion from; a datum value and-in which the mag nitude "oithe same physical condition may be varied by.-'contro1 means having :a rnovable nem ber; the.- -cor nbination :of r g-transmitter resnons'ive to 'said physicaln condition and =producing-an}elec. *tri'c current whose value. is proportional-to"the. magnitude of said condition, acircuit arrange merit for producing a currentQcomponent ons forming j with the -.-integral with respect to time of the actual f devi-ation of the magnitude of said physical-condition from its datum value, said cir. cuit arrangement comprising abridge circuit-hay: ing; two fphotocells connectedin adjacent arrns thereofpmearis;forsupplying a polarizing yoltage to said cells, .me ans responsive to the cur-rent from said transmitter for controlling thediffer ential, illumination-Tot said cells: in.;-accordance" with the: deviation from its datum value 1 of; the curieri'tiirfomsaid transmitter,;a condenserconnected to be chargedby theouteof-bfilarice curlrent oisaid bridge, toa voltage, representing the integ-ralrwith respect, to time-of the deviation of the current-from said transmitter, ;and-a vacuum tube relay ihaving its input circuit icontrolled l by the volt-age across I saidecondenser and producing in its-output circuit the required current com ponent; and. a current-operated :control dei ice. connected :to operatesaid movable member and connected. forenergizati-on bvthe :sumof the. current from fsaid transmitter and =saidintegral current oomponent. i i r i 5. A -system according to Clalm fl; in which said meansyior :controlling: the :diiierential illurninati-on: Off.lih8. photocell; con pri ses ,an electrical measuring iiinstmmeni i .ner i ed fbyits d t ans-g mitte'r currentgfin ghavingga .nioving. elementa source of illumination, and shutter means actuated by said moving element for controlling the proportion of the light from said source falling on each of said photo-cells.

6. A system according to claim wherein the moving element of said measuring instrument comprises a moving coil connected to be energized by said transmitter current, and wherein said shutter is positioned midway between said photocells when said transmitter current is at its datum value, said shutter being operative upon deviation of said transmitter current from its datum value to vary the amount of light transmitted to said cells in opposite directions.

7. A system according to claim 4 and including means for adjusting the intensity of illumination of said photo-cells and thereby varying the magnitude of the integral current component in relation to that of the proportional current component.

8. A system according to claim 4 and including means for applying to the energizing circuit for said control device a derivative current component which conforms with the rate of change of the magnitude of said proportional current and is applied in a direction opposite to the proportional current component.

9. A system according to claim 4 and including a difierentiating electrical network connected in the path of the proportional current component and deriving a voltage proportional to the first derivative of said proportional current, and a connection for supplying said derivative voltage to the input circuit of said vacuum tube relay.

10. A system according to claim 9 and including a resistance connected in series with said condenser in the input circuit of said vacuum tube relay, and in which said connection applies said derivative voltage across said resistance, whereby the voltage across said resistance and across said condenser are algebraically additive.

11. A system according to claim 9 and including a resistance connected in the circuit of said proportional current component, and wherein said difierentiating network is energized by voltage derived from said resistance, and including means for varyingthe value of said derived voltage.

12. An integrating circuit arrangement comprising a bridge circuit having two photo-cells connected in adjacent arms thereof, means for supplying a polarizing voltage to said cells, means for controlling the difierential illumination of said cells in accordance with the deviation from its datum value of a variable physical condition, a condenser connected to be charged by the outof -balance current of said bridge to a voltage representing the integral with respect to time of the 12 wherein said means for controlling the differential illumination of said cells comprises a movable shutter normally positioned midway between said cells and controlling the amount of light admitted to said cells, and means responsive to said variable physical condition for shifting said shutter to one side or the other of said normal position in accordance with an increase or decrease in the magnitude of said condition from its datum value.

14. In a regulator system in which the magnitude of a physical condition is subject to variation from a datum value and in which the magnitude of the same physical condition may be varied by control means having a movable member, the combination of a transmitter responsive to said physical condition and producing an electric current whose value is proportional to the magnitude of said condition, a circuit arrangement for producing a current component conforming with the integral with respect to time of the actual deviation of the magnitude of said physical condition from its datum value, said circuit arrangement comprising a normally balanced four-arm bridge arrangement having a source of polarizing voltage connected between two diagonal terminals thereof and a null circuit connected between the two remaining diagonal terminals, means responsive to the proportional current from said transmitter for unbalancing said bridge in accordance with the deviation of said proportional current from its datum value, a condenser connected to be charged by the current in the null circuit of said bridge to a volt age representing the integral with respect to time of the deviation of the proportional current from its datum value, and a vacuum tube relay having its input circuit controlled by the voltage across said condenser and producing in its output circuit the required current component; and a current-operated control device connected to operate said movable member and connected for energization by the sum of the current from said transmitter and said integral current component.

WILLIAM THOMAS MARCHMENT. JAMES REILLY BOUNDY. STEPHEN ARCHBOLD BERGEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,497,401 Boddie June 10, 1924 2,117,894 Lenehan May 17, 1938 2,172,487 Wasserlein Sept. 12, 1939 2,173,810 Taylor Sept. 19, 1939 2,175,985 Callender et al Oct. 10, 1939 2,268,285 Ivanofi Dec. 30, 1941 2,282,726 Jones May 12 1942 2,300,537 Davis Nov. 3, 1942 2,312,711 Harrison Mar. 2, 1948 2,313,079 Lilja Mar. 9, 1943 2,390,793 Jones Dec. 11, 1945 

